1. Field of the Invention
This invention relates to the field of fire detection and, more particularly, to a fire detection system responsive to the unique spectral energy characteristic of a hydrocarbon flame.
2. Prior Art
The use of light responsive photocells in conjunction with wavelength selective filters for the discrimination and detection of a particular light source from among several sources is known to the prior art. However, flame detection systems known in the prior art have generally operated on the principle of detecting one or more bands in the spectrum which are characteristic of a particular type of flame. Typically, two or more identical photocells are utilized together with multi-layer wavelength selective filters, the latter being produced especially for the application by vapor deposition techniques. Since the photocells are identical and have substantially the same response, the attainment of wavelength selectivity must perforce be due entirely to the design of the filters. Consequently, the cost of such flame detection systems is relatively expensive, due in large measure to the requirement for specially designed filters.
The present invention overcomes the foregoing shortcomings by teaching the use of two different photovoltaic devices; having respective spectral responses in different photovoltaic portions of the spectrum. Using two different photocells, e.g., silicon and selenium photovoltaic cell diodes, this invention operates on the principle of detecting the presence of spectral energy in the band of one of the photocells, and the absence of significant spectral energy in the band of the other. By this technique, the use of commercially available off-the-shelf filters is enabled, thereby reducing the cost of the invention system substantially below that of systems heretofore known; e.g., relatively inexpensive Wratten gelatin filters and other chemical filters are suitable for use in this invention. Moreover, notwithstanding the significant economic advantage of the invented system with respect to systems of the prior art, the invented system has superior performance characteristics.
One flame detection system of the prior art utilizes a gas filled Geiger-Muller tube. It operates by detecting the rich ultraviolet light energy characteristic of a flame (in the 0.2 to 0.25 micron range). This system has the disadvantage of being both relatively expensive and also slow in response time. In the latter connection, systems utilizing a Geiger-Muller tube typically discriminate on the basis of the rate of an output pulse train. This requires an integration over a period of time, typically 30 seconds, before a response is forthcoming. Still another flame detection system of the prior art utilizes a pair of cadmium sulfide photocells in an electrical bridge configuration. The cadmium sulfide photocells are relatively expensive, photo-conductive devices. Moreover, they are limited by a relatively narrow spectral response characteristic. The present invention does not suffer from the shortcomings and disadvantages of flame detection systems which utilize Geiger-Muller tubes and cadmium sulfide photocells.